This invention relates to a memory-type automatic adjustment system, and more particularly, to such a system wherein a particular parameter of a signal, such as the gain of a signal, is automatically controlled. A preferred application of the present invention is in a white balance adjustment circuit for a video camera.
In some automatic adjustment systems, an input signal or signals is controlled in accordance with a predetermined or desired condition. For example, in an automatic gain control system, the gain of an input signal is controlled in accordance with a reference level such that if the input signal level or the reference level varies, a corresponding adjustment is made in the gain of the controlled signal. Often, the condition upon which an adjustment is made is not continuous. Rather, a determination of a proper control signal may be made during one time period and this determined control signal then may be stored for future time periods in order to effect an automatic adjustment operation.
The foregoing type of an automatic adjustment or control operation advantagesously utilizes a memory-type control device. One such use of the memory-type control device is in an automatic white balance adjustment system for a video camera. In a color television camera, a proper white balance is achieved when the individual color signals are of predetermined amplitudes, or levels, when a white object is viewed by the camera. However, even if the white balance of the camera is set or adjusted to be correct for one color temperature of an object, this white balance may be lost or otherwise degraded in the event that the color temperature of illumination for the object changes. A loss of the white balance condition also may arise when the operating temperature of the electrical apparatus included in the camera changes, or due to other changes in the electrical components over long periods of time. Accordingly, to account for these changes, it often is necessary to update the white balance of the color video signals, and then to use the updated, balanced signals to control the respective gains of the color signals.
In controlling the white balance of color video signals, a white object is viewed, for example, a white wall, board, or other reference object, and the gains, or levels, of the respective color signals which are produced in response to the viewed white object are adjusted until proper white balance is attained. The control signals which ultimately result in the proper white balance are stored and are used as gain control signals when the camera apparatus views other objects, such as a televised scene. When the white balance changes, or is expected to change, because of the above-mentioned conditions, the gain control signals are updated by performing again the aforementioned white balance operation.
One type of white balance control system which is effective in controlling the white balance of color video signals is disclosed in U.S. Pat. No. Re. 28,774. In that system, color difference signals, such as the red (R-Y) and blue (B-Y) color difference signals are integrated and supplied to respective memory circuits for controlling the gains of the red and blue video signals which are produced by the color camera. When proper white balance is attained, the color difference signals R-Y and B-Y each are zero. This zero level establishes the proper gain of the respective red and blue gain control circuits. In the event that the white balance is lost, a corresponding color difference signal voltage is produced and is used to adjust the gains of the red and blue signals in a direction so as to restore the white balance. The signal levels which are used to establish the gains of the red and blue gain control circuits so as to provide proper white balance are stored. Hence, when the camera is used to televise a scene other than a reference white object, the televised scene likewise will have a proper white balance in accordance with the stored control signals. In another embodiment of the white balance control system disclosed in this patent, red-green (R-G) and blue-green (B-G) color difference signals are used to determine whether proper white balance of a viewed white reference object is achieved. That is, the R-G and B-G signals are used to adjust the respective gains of the red and blue signals such that the difference between the gain-controlled red and blue signals and the green signal result in correct white balance. The control signals which attain this proper balance are stored and used to establish the gains of the red and blue signals when the video camera apparatus is used to view a televised scene.
In yet another type of white balance control system, white balance is achieved by using the camera to view a white reference object and then to control the level of the luminance component Y in such a manner that the color difference signals R-Y ad B-Y, derived from the viewed white object, each are zero. The control signals which result in proper white balance then are stored and used to control the gain of the luminance component for further usage of the video camera.
In the white balance control systems of the type described above, the control signals, such as the gain control signals, which must be stored and used when the camera televises a normal scene generally are stored by a memory circuit constituted by a capacitor. However, as is known, a capacitor usually is accompanied by leakage, whereby the level of a signal stored thereacross is degraded, or reduced, over prolonged periods of time. Hence, although the control signal stored across the capacitor initially will be correct to achieve proper white balance, the leakage of this signal soon will result in a loss of white balance. This means that frequent readjustments in white balance are necessary.
Another type of memory-type white balance control system is described in NEC Research and Development Bulletin, No. 40, January 1976, wherein an UP/DOWN counter is used to derive and store a digital control signal which is converted to an analog gain control signal. However, this publication does not describe how the UP/DOWN counter is controlled other than by suggesting that red-green and blue-green difference signals are used.